1. Field of the Invention
The present invention relates to a temperature indicating material suited for the formation of a temperature monitoring label or the like used for the monitoring of an environmental temperature. This invention relates to a temperature monitoring member which is produced using a temperature indicating member which undergoes color changes with temperature and is used for the temperature monitoring of vegetables and fruits, perishable foods and the like.
2. Description of the Prior Art
As a temperature indicating material which undergoes color changes with temperature, thermochromic organic pigments, for example, ethylene derivatives substituted with a condensed aromatic ring such as spiropirans bianthrone or dixanthylene, crystals of a metal complex salt composed of CoCl2xc2x72 (CH2)6N4xc2x710H2O, combinations of an electron donating compound, an electron accepting compound and a polar organic compound and the like have conventionally been put on the market. Upon use of a temperature monitoring label prepared from such a temperature indicating material, color changes with temperature are judged visually.
Roughly speaking, there are two types of temperature indicating materials, that is, a reversible type and irreversible type. In the former one, the color changes unlimitedly in response to a temperature increase or decrease, while in the latter one, the color which has once changed at a certain temperature does not show any further change.
The reversible type material is used for indicating the time good for drinking of beer or the like, or for visually attracting attentions to a temperature change.
The irreversible type material is used for the temperature monitoring of fruits and vegetables, or perishable foods. The temperature indicating material which has stored its thermal history is disclosed in Japanese Patent Publication No. SHO 58-10709, wherein an ink having a viscosity changeable with temperature (for example, an ink which has a pigment dissolved in oleyl alcohol) penetrates in a penetration material and the length of the ink penetrated in the material indicates how long it is exposed to a temperature not lower than a predetermined temperature. In addition, there is an irreversible temperature indicating material for temperature monitoring which has a plurality of layers formed on a label. These plurality of layers are an ink layer, a separate layer, a porous layer and an indicator and they are stacked one after another. The separate layer is removed when the temperature monitoring is started, while the porous layer controls the penetration time of the ink from the ink layer until it reaches the indicator. For such a temperature monitoring member, a storing device (separator or the like) prior to the starting of temperature monitoring is important, but it is expensive and in addition, has such a complex structure that a desired printing pattern cannot be adopted. Moreover, this material needs a cumbersome operation such as removal of the separator or the like upon initiation of the temperature monitoring.
As an improved process, a process using a rewritable material is disclosed in Japanese Patent Application Laid-Open No. 197853/1996, wherein the glass transition point of the material is used as a preset temperature and temperature monitoring is effected by making use of the fact that a drastic change in the diffusion rate of molecules occurs at a temperature exceeding the glass transition point.
The phase separation controller described in this Japanese Patent Application Laid-Open No. 197853/1996 is employed for accelerating the color developing rate at high temperatures, but cannot improve the properties necessary as thermochromic characteristics (characteristics of accelerating color development at a temperature not lower than a preset temperature in spite of slow color development at a temperature not higher than the preset temperature).
The problems of such a prior art will next be described. In the process using a rewritable material, a system which shows a drastic change in a color developing rate at a temperature not lower than the glass transition point after initial decolorization is utilized. The temperature monitoring material in this case has a mechanism of developing color owing to the association of a developer which has been diffused as a result of the phase separation between a reversal material and the developer with a leuco dye so that color development gradually proceeds even at a temperature not higher than the glass transition point. A density change with temperature is however small, so it is necessary to heighten the sensitivity in practical use. The phase separation controller for raising the color developing rate at high temperatures cannot serve as a temperature indicating material, since it has a high color developing rate even at low temperatures. In addition, the color density upon decolorization is not complete and a density ratio (S/N ratio) of decolorized state to color-developed state by temperature is low.
An object of the present invention is therefore to provide a temperature indicating material which has thermochromic characteristics sensitive at an environmental temperature and has a high S/N ratio.
In one aspect of the present invention, there is thus provided a temperature indicating material which comprises an electron donating compound, an electron accepting compound and a reversal material and undergoes color change with temperature and time, characterized in that said temperature indicating material further comprises a thermochromism controller which plays a role of changing a rate from crystal to amorphous or from phase separation to non-phase-separation and upon discoloration by an environmental temperature, crystallizes or undergoes phase separation to provide a place for reaction, thereby contributing to the discoloration of the temperature indicating material.
In another aspect of the present invention, there is also provided a temperature monitoring member which is prepared using a temperature indicating member obtained by incorporating, in a material which comprises an electron donating compound, an electron accepting compound and a reversal material, is initialized upon initiation of temperature monitoring and undergoes color changes with temperature and time, a cycloalkanol or cycloalkane diol which has stable crystalline conditions within a temperature monitoring range; and by printing or coating information to a base material.